A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a factory, commonly referred to as a “fab” or “foundry”, in which semiconductor or other devices are manufactured, each lithographic apparatus is commonly grouped with a “track” comprising substrate handling devices and pre- and post-processing devices to form a lithographic processing cell commonly referred to as “lithocell”. Substrates (e.g., wafers), which may be blank or have already been processed to include one or more process or device layers, are delivered to the lithocell in lots (also referred to as batches) for processing. A lot is, in general, a group of substrates which are to processed by the lithocell in the same way and is accompanied by a “recipe” which specifies the processes to be carried out. The lot size may be arbitrary or determined by the size of carrier used to transport substrates around the fab. The recipe may include details of the resist coating to be applied, temperature and duration of pre- and post-exposure bakes, details of the pattern to be exposed and the exposure settings for that, development duration, etc.
Inevitably, errors occur in the processing of substrates, e.g. focus spots caused by dust on the substrate table and overlay errors, and various measurement techniques, collectively referred to as metrology, for detecting them are known. If an error can be detected before an irreversible process step, e.g. an etch, has been carried out, the developed resist may be stripped from the substrate and the substrate reprocessed with the aim of producing a correct layer. Existing metrology devices are generally off-line, that is stand alone devices outside the lithocell to which exposed substrates are taken to be measured. The measurement processes may be time consuming, so that often only a sample of the substrates in a lot are measured and in the event that errors are detected the whole lot is stripped and reprocessed. This can be inefficient in that many perfectly good substrates may be reprocessed and ineffective in that the sampling of substrates may not detect a few bad substrates in a lot.